A magnetic random access memory (MRAM) is a non-volatile memory which basically includes a magnetoresistive (MR) material, a sense line, and a word line. The MRAM employs the magnetic vectors to store memory states. Magnetic vectors in one or all of the layers of MR material are switched very quickly from one direction to an opposite direction when a magnetic field is applied to the MR material over a certain threshold. According to the direction of the magnetic vectors in the MR material, states are stored, for example, one direction can be defined as a logic "0", and another direction can be defined as a logic "1". The MR material maintains these states even without a magnetic field being applied. The states stored in the MR material can be read by passing a sense current through the cell in a sense line because of the difference between the magnetic resistances of the two states.
Magnetic tunneling junction (MTJ) structure or cells include at least a pair of magnetic layers with a non-magnetic, non-conducting tunnel layer (commonly referred to as the barrier layer) sandwiched therebetween. When a sense voltage is applied between the pair of magnetic layers, electrical carriers travel between the pair of magnetic layers by tunneling through the non-magnetic, non-conducting tunnel layer sandwiched between the magnetic layers. The resistance to the sense current is a maximum when the magnetic vectors of the pair of magnetic layers are anti-parallel and minimum when the magnetic vectors of the pair of magnetic layers are parallel. The difference between the maximum and minimum resistance is commonly referred to as the magnetoresistance (MR) ratio.
The minimum resistance of the MTJ cell (commonly referred to as the areal resistance) is determined by the construction and materials of the MTJ cell. Clearly, in an ideal MTJ cell the areal resistance would be very low or zero and the maximum resistance would be very high or infinite, similar to an ideal switch. In the prior art, the primary method used to reduce the resistance of an MTJ cell is to make the barrier layer thinner. The cell resistance problem is further aggravated by the fact that in very high density arrays of magnetic memory cells the size of individual cells becomes very small (submicron) if the array is constructed small enough to be useful in present day electronic devices. Thus, to make the barrier layers "thinner", they must become ultra thin, or in a range of a few angstroms thick.
Attempts to reduce the barrier layer to ultra thin dimensions have not been successful because ultra thin layers are usually not continuous, i.e. contain pin holes and the like. For example, one prior art attempt to reduce the areal resistance includes depositing a layer of pure aluminum on the lower magnetic layer of the MTJ and then oxidizing the aluminum layer in oxygen plasma. A problem with this procedure is that as the aluminum layer is deposited, pinholes tend to form, especially if the layer is thin. As the aluminum is oxidized, some of the pinholes tend to remain and produce shorts in the MTJ cell when the second magnetic layer is deposited on the aluminum oxide layer. To overcome the pinhole problem, one possible solution suggested by the prior art is to deposit the aluminum layer at low temperatures (e.g. the temperature of liquid nitrogen), thereby, reducing the size of the grains. Some of the problems with this method are that it involves extensive heating and cooling cycles, takes a long time, costs more and hence is not a method which can be used in manufacturing.
Some additional problems which arise with attempts to produce thinner layers are that the thinner layers are hard to make reproducibly, and they possibly have issues of thermal stability and electrical breakdown.
Accordingly, it is highly desirable to provide magnetic tunneling junctions for magnetic random access memories which include barrier layers with reduced resistance.
It is a purpose of the present invention to provide new and improved MTJ cells with reduced resistance and methods of fabrication.
It is another purpose of the present invention to provide new and improved MTJ cells and a method of fabrication with high quality barriers or tunnel layers.
It is still another purpose of the present invention to provide new and improved MTJ cells with a high magnetoresistance (MR) ratio.
It is a further purpose of the present invention to provide a new and improved method of fabricating MTJ cells which is easily adaptable to manufacturing.